The SIG SG 550 is an assault rifle manufactured by SIG Sauer AG (formerly a division of Schweizerische Industrie Gesellschaft, now known as SIG Holding AG) in Switzerland. "SG" is an abbreviation for Sturmgewehr ("assault rifle"). The SG 550 was co-developed by Sig Sauer AG and Italian firearm manufacturer Beretta, and is based on the earlier predecessor, the SIG SG 540, chambered in 5.56×45mm NATO.
On a firearm or other tools, a pistol grip is a distinctly protruded handle underneath the main mechanism, to be held by the user's hand at a more vertical (and thus more ergonomic) angle, similar to how one would hold a conventional pistol.
In firearms, the pistol grip is located behind the trigger and generally held by the hand that operates the trigger. Rifles and shotguns without pistol grips are generally referred to as having "straight" or "upland" (shotguns only) style stocks. Some firearms, starting from a 1840s Belgian carbine, and some automatic weapons in the 20th century (e. g., Chauchat MG, Thompson submachine gun, AK-47 assault rifle), have a second frontal pistol grip (or foregrip) on the firearm's fore-end to be used by the support hand for better stability in operation.
Select fire is the capability of a weapon to be adjusted to fire in semi-automatic, fully automatic, or burst mode. The modes are chosen by means of a fire selector switch, which varies depending on the weapon's design. Some select-fire weapons have burst fire mechanisms to limit the maximum number of shots fired automatically in this mode. The most common limits are two or three rounds per trigger pull. Fully automatic fire refers to the ability for a weapon to fire continuously until either the feeding mechanism is emptied or the trigger is released. Semi-automatic refers to the ability to fire one round per trigger pull.
The presence of select fire modes on firearms permits more efficient use of rounds to be fired for specific needs, versus having a single mode of operation, such as fully automatic, thereby conserving ammunition while maximizing on-target accuracy and effectiveness. This capability is most commonly found on military weapons of the 20th and 21st centuries.
In automatic firearms, burst mode or burst-fire is a firing mode enabling the shooter to fire a predetermined number of rounds, usually two or three rounds on hand held weapons to fifty or more rounds on autocannons, with a single pull of the trigger. This firing mode is commonly used in submachine guns and assault rifles. Other types of firearms, such as machine pistols, including the Heckler & Koch VP70 and Beretta 93R also have a burst mode.
The burst mode is normally employed as an intermediate fire mode between semi-automatic and fully automatic, although some firearms lack a "full auto" capability and use a burst mode instead. For instance, the M16A4 (the standard-issue service rifle of the USMC) has, in addition to the semi-automatic mode, a three-round burst mode, which replaced the fully automatic mode of the previous M16A1 and M16A3 models. The reason for this replacement was the massive waste of ammunition and very poor performance of soldiers who fired their rifles in fully automatic mode during the Vietnam War.